The present invention relates to catalysts for the polymerization of olefins. In particular, it relates to high activity catalysts for the polymerization of olefins obtained from cyclopentadienyl compounds of a transition metal, organometallic aluminium compounds and water.
The invention also relates to processes for the polymerization of olefins carried out in the presence of the catalysts of the invention.
Homogeneous catalytic systems based on metallocene compounds, aluminium alkyl compounds and water are known to be active in the polymerization of olefins.
In European Patent Application EP 384,171, catalysts for the polymerization of olefins are described which comprise the reaction product of:
(a) a metallocene compound of formula:
(C5Rxe2x80x2n)mRxe2x80x3p(C5Rxe2x80x2n)MX3xe2x88x92m
wherein (C5Rxe2x80x2n) is an optionally substituted cyclopentadienyl group and two or four Rxe2x80x2 substituents of one and the same cyclopentadienyl group can form one or two rings having 4 to 6 carbon atoms; Rxe2x80x3 is a divalent radical bridging the two cyclopentadienyl groups; X can be for instance an halogen atom; M is a transition metal selected from Ti, Zr and Hf; p is 0 or 1; m is 0, 1 or 2; when m=O, then p=0 and when p=0, at least one Rxe2x80x2 substituent is different from hydrogen; n=4 when p=1 and n=5 when p=0; and
(b) an alumoxane of the formula: 
xe2x80x83wherein the substituents R can generically be alkyl, alkenyl or alkylaryl radicals, having 2-20 carbon atoms.
The alumoxanes (b) are prepared by reacting the corresponding trialkylaluminium compounds with water in a molar ratio of 2:1. In the embodiment examples, alumoxanes are used in which the R substituents are ethyl, isobutyl or 2-methylpentyl groups.
European Patent Application EP 575,875 describes homogeneous catalytic systems for the polymerization of olefins comprising:
(A) a cyclopentadienyl compound of the formula:
(C5R1xxe2x88x92mH5xe2x88x92x)R2m(C5R1yxe2x88x92mH5xe2x88x92y)nMQ3xe2x88x92n
in which M is Ti, Zr or Hf; C5R1xxe2x88x92mH5xe2x88x92x and C5R1yxe2x88x92mH5 are equally or differently substituted cyclopentadienyl rings and two or four substituents R1 of the same cyclopentadienyl group can form one or two rings, having from 4 to 6 carbon atoms; R2 is a bridging group which links the two cyclopentadienyl rings; the substituents Q are preferably chlorine atoms; m can be 0 or 1; n can be 0 or 1, being 1 when m=1; x is an integer comprised between m+1 and 5; y is an integer comprised between m and 5;
(B) an organometallic aluminium compound of formula:
AlR43xe2x88x92Hz
wherein the substituents R4 are alkyl, alkenyl or alkylaryl radicals containing from 1 to 10 carbon atoms, optionally containing Si or Ge atoms, at least one of the substituents R4 being different from a straight alkyl group, z is 0 or 1; and
(C) water.
The molar ratio between the organometallic aluminium compound (B) and water (C) is comprised between 1:1 and 100:1. In the embodiment examples, only triisobutylaluminium and triisohexylaluminium are used as the organometallic aluminium compounds.
International Patent Application WO96/02580 describes improved catalytic systems comprising:
(A) a cyclopentadienyl compound of formula:
(C5R1xxe2x88x92mH5xe2x88x92x)R3m(C5R1yxe2x88x92mH5xe2x88x92y)nMQ3xe2x88x92n
in which M is Ti, Zr or Hf; C5R1xxe2x88x92mH5xe2x88x92x and C5R1yxe2x88x92mH5xe2x88x92y are cyclopentadienyl rings substituted in the same way or different ways; the substituents R1, the same or different from each other, are alkyl, alkenyl, aryl, alkylaryl or arylalkyl radicals, which have 1 to 20 carbon atoms and optionally contain Si or Ge atoms or Si(CH3)3 groups, or two or four substituents R1 of one and the same cyclopentadienyl group can also form one or two rings, having 4 to 6 carbon atoms; R2 is a bridging group linking the two cyclopentadienyl rings and is selected from CR32, C2R34, SiR32, Si2R34, GeR32, Ge2R34, R32SiCR32, NR1 and PR1, wherein the substituents R3, the same or different from each other, are R1 or hydrogen, or two or four substituents R3 can also form one or two rings having 3 to 6 carbon atoms; the substituents Q, the same or different from each other, are halogen, hydrogen, R1, OR1, SR1, NR12 or PR12; m can be 0 or 1; n can be 0 or 1, being 1 if m=l; x is an integer ranging from (m+1) to 5; and y is an integer of ranging from m to 5;
(B) an organometallic aluminium compound of formula:
Al(CH2xe2x80x94CR4R5R6)wR7yHz
wherein in the (CH2xe2x80x94CR4R5R6) groups, the same or different from each other, R4 is an alkyl, alkenyl or arylalkyl group, having from 1 to 10 carbon atoms; R5 is an alkyl, alkenyl, aryl, arylalkyl or alkylaryl group, having from 3 to 50 carbon atoms, which is different from a straight alkyl or alkenyl group and, optionally, R4 and R5 fused together can form a ring having from 4 to 6 carbon atoms; R6 is hydrogen or an alkyl, alkenyl or arylalkyl group, having from 1 to 10 carbon atoms; the R7 substituents, the same or different from each other, are alkyl, alkenyl, aryl, arylalkyl or alkylaryl radicals, containing from 1 to 10 carbon atoms, optionally containing Si or Ge atoms; w is 1, 2 or 3; z is 0 or 1; y=3xe2x88x92wxe2x88x92z; and
(C) water.
The molar ratio between the organometallic aluminium compound (B) and water (C) is comprised between 1:1 and 100:1. The only organometallic aluminium compounds disclosed are those wherein the (CH2xe2x80x94CR4R5R6) groups are 2,4,4-trimethyl-pentyl, 2-phenyl-propyl or 1-butene oligomers.
However, the catalysts described in the above-cited patent applications exert activities in the polymerization of olefins which are not completely satisfactory. Therefore, the problem addressed by the present invention is to improve the activities of the above-discussed known catalysts.
To solve this problem, novel catalysts have unexpectedly been found which are suitable for the polymerization of olefins and possess a considerably improved activity compared with the known catalysts.
Thus, according to a first object, the present invention provides a catalyst for the polymerization of olefins comprising the product obtained by contacting the following components:
(A) a cyclopentadienyl compound of formula (I):
(C5R1xxe2x88x92mH5xe2x88x92x)R2m(C5R1yxe2x88x92mH5xe2x88x92y)nMQ3xe2x88x92nxe2x80x83xe2x80x83(I)
wherein M is Ti, Zr or Hf; C5R1xxe2x88x92mH5xe2x88x92x, and C5R1yxe2x88x92mH5xe2x88x92y are equally or differently substituted cyclopentadienyl rings; the substituents R1, the same or different from each other, are selected from the group consisting of a linear or branched, saturated or unsaturated C1-C20 alkyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl or C7-C20 arylalkyl groups radicals, optionally containing Si or Ge atoms or Si(CH3)3 groups, or two or four substituents R1 of the same cyclopentadienyl group form one or two rings, having 4 to 6 carbon atoms; R2 is a bridging group between the two cyclopentadienyl rings and is selected from the group consisting of CR32, C2R34, SiR32, Si2R34, GeR32, Ge2R34, R32SiCR32, NR1 and PR1, wherein the substituents R3, the same or different from each other, are hydrogen or have the same meaning of R1, or two or four substituents R3 form one or two rings, having 3 to 6 carbon atoms; the substituents Q, the same or different from each other, are selected from the group consisting of halogen, hydrogen, R1, OR1, SR1, NR12 and PR12; m is 0 or 1; n is 0 or 1, being 1 when m=1; x ranges from (m+1) to 5; and y ranges from m to 5;
(B) an organometallic aluminium compound of formula (II):
Al(CH2xe2x80x94CR4R5xe2x80x94CR6R7R8)wR9qHzxe2x80x83xe2x80x83(II)
wherein R4 is a linear or branched, saturated or unsaturated C1-C10 alkyl or C7-C20 arylalkyl group; R5 is hydrogen or a linear or branched, saturated or unsaturated C1-C10 alkyl or C7-C10 arylalkyl group; R6 and R7, the same or different from each other, are linear or branched, saturated or unsaturated C1-C10 alkyl, C6-C10 aryl, C1-C10 arylalkyl or alkylaryl groups; the substituents R4 and R6 and/or R6 and R7 optionally form one or two rings, having 3 to 6 carbon atoms; R8 is hydrogen or a linear or branched, saturated or unsaturated C1-C10 alkyl, C6-C10 aryl, C7-C10 arylalkyl or C7-C10 alkylaryl group; R9 is a linear or branched, saturated or unsaturated C1-C10 alkyl or C1-C10 arylalkyl group, a carbon atom in the compound of formula (II) being optionally replaced by a Si or a Ge atom; w is 1, 2 or 3; z is 0 or 1; q=3xe2x88x92wxe2x88x92z; and
(C) water.
The molar ratio between the organometallic aluminium compound (B) and water (C) preferably ranges from 1:1 to 100:1, and more preferably from 1:1 to 50:1. A suitable value for the Al/H2O molar ratio is 2.
The molar ratio between the organometallic aluminium compound (B) and the cyclopentadienyl compound (A), calculated as Al/M molar ratio, preferably ranges from 50 to 50000, and more preferably from 500 to 5000.
In the cyclopentadienyl compounds of formula (I), the metal M is preferably zirconium. When in formula (I) m=0, C5R1xxe2x88x92mH5xe2x88x92x and C5R1yxe2x88x92mH5xe2x88x92y are preferably pentamethylcyclopentadienyl, indenyl or 4,5,6,7-tetrahydroindenyl; the Q substituents are preferably chlorine atoms or C1-C7 hydrocarbyl groups, and more preferably are methyl groups.
Non-limiting examples of cyclopentadienyl compounds of formula (I), wherein m=0, are:
wherein Me=methyl, Et=ethyl, Cp=cyclopentadienyl, Ind=indenyl, H4Ind=4,5,6,7-tetrahydroindenyl, Benz=benzyl; M is preferably Zr.
When in formula (I) m=1, C5R1xxe2x88x92mH5xe2x88x92x and C5R1yxe2x88x92mH5xe2x88x92y are preferably tetramethyl-cyclopentadienyl, indenyl, 4,5,6,7-tetrahydroindenyl, 2-methyl-4,5,6,7-tetrahydroindenyl, 4,7-dimethyl-4,5,6,7-tetrahydroindenyl, 2,4,7-trimethyl-4,5,6,7-tetrahydroindenyl or fluorenyl groups; preferably (CH3)2Si less than  or xe2x80x94CH2CH2xe2x80x94; the Q substituents are preferably chlorine or C1-C7 hydrocarbyl groups, and more preferably methyl groups.
Non-limiting examples of cyclopentadienyl compounds of formula (I), wherein m=1, are:
wherein Me=methyl, Cp=cyclopentadienyl, Ind=indenyl, Flu=fluorenyl, Ph=phenyl, H4Ind=4,5,6,7-tetrahydroindenyl; M is preferably Zr.
In the organometallic aluminium compounds of formula (II), R4 is preferably a C1-C5 alkyl group, more preferably a C1-C3, alkyl group, and even more preferably a methyl or ethyl group; R5 is preferably hydrogen; R6 and R7 are preferably C1-C5 alkyl groups, more preferably C1-C3 alkyl groups; R8 is preferably hydrogen or a C1-C5 alkyl group, more preferably a C1-C3 alkyl group; w is preferably 2 or 3, more preferably 3; when q is different from 0, R9 is preferably a C1-C5 alkyl group, more preferably a branched alkyl group, even more preferably isobutyl.
According to the invention, component (B) can suitably comprise a mixture of two or more organometallic aluminum compounds of formula (II). Moreover, Component (B) can be used in combination with other organometallic aluminum compounds, other than those of formula (II), or in mixture with other compatible cocatalysts known in the state of the art.
In the organometallic aluminium compounds of formula (II), z is 0 or 1. As it is known in the state of the art, aluminium trisalkyls may contain small amounts of bisalkyl-aluminium hydride; the hydride content can slightly change during prolonged storage periods and depending on the storage temperature. Therefore, according to a preferred embodiment of the invention, component (B) is a mixture of the two organometallic aluminium compounds of formula (II) wherein z=0 and z=1, so that the molar ratio between the hydrogen atoms directly bound to aluminium and aluminium atoms (i.e. the overall z value) is lower than 0.8, and even more preferably ranges from 0.05 to 0.3. Mixtures of organometallic aluminium compounds having said overall z values can be prepared with methods known in the state of the art, for instance by mixing the corresponding trisalkylaluminium and bisalkylaluminium hydride in appropriate molar ratios.
Non-limiting examples of organometallic aluminium compounds (B) of formula (II), according to the present invention, are:
tris(2,3,3-trimethyl-butyl)aluminium,
tris(2,3-dimethyl-hexyl)aluminium,
tris(2,3-dimethyl-butyl)aluminium,
tris(2,3-dimethyl-pentyl)aluminium,
tris(2,3-dimethyl-heptyl)aluminium,
tris(2-methyl-3-ethyl-pentyl)aluminium,
tris(2-methyl-3-ethyl-hexyl)aluminium,
tris(2-methyl-3-ethyl-heptyl)aluminium,
tris(2-methyl-3-propyl-hexyl)aluminium,
tris(2-ethyl-3-methyl-butyl)aluminium,
tris(2-ethyl-3-methyl-pentyl)aluminium,
tris(2,3-diethyl-pentyl)aluminium,
tris(2-propyl-3-methyl-butyl)aluminium,
tris(2-isopropyl-3-methyl-butyl)aluminium,
tris(2-isobutyl-3-methyl-pentyl)aluminium,
tris(2,3,3-trimethyl-pentyl)aluminium,
tris(2,3,3-trimethyl-hexyl)aluminium,
tris(2-ethyl-3,3-dimethyl-butyl)aluminium,
tris(2-ethyl-3,3-dimethyl-pentyl)aluminium,
tris(2-isopropyl-3,3-dimethyl-butyl)aluminium,
tris(2-trimethylsilyl-propyl)aluminium,
tris(2-methyl-3-phenyl-butyl)aluminium,
tris(2-ethyl-3-phenyl-butyl)aluminium,
tris(2,3-dimethyl-3-phenyl-butyl)aluminium,
tris(1-menthen-9-yl)aluminium,
as well as the corresponding compounds wherein one of the hydrocarbyl groups is replaced by a hydrogen atom, and those wherein one or two of the hydrocarbyl groups are replaced by an isobutyl group. Particularly preferred compounds are tris(2,3,3-trimethyl-butyl)aluminium and tris(2,3-dimethyl-butyl)aluminium.
The components of the catalysts of the present invention can be brought into contact in different ways. It is possible for instance, to contact first the aluminium compound (B) with water (C) and subsequently to bring the thus obtained reaction product into contact with the cyclopentadienyl compound (A).
Therefore, a preferred embodiment of the present invention is a catalyst for the polymerization of olefins comprising the product obtained by contacting the following components:
(A) a cyclopentadienyl compound of formula (I):
(C5R1xxe2x88x92mH5xe2x88x92x)R2m(C5R1yxe2x88x92mH5xe2x88x92y)nMQ3xe2x88x92nxe2x80x83xe2x80x83(I)
wherein M, C5R1xxe2x88x92mH5xe2x88x92x, C5R1yxe2x88x92mH5xe2x88x92y, R2, Q, m and n have the meaning reported above; and
(Bxe2x80x2) the product of the reaction between water and an organometallic aluminium compound of formula (II):
Al(CH2xe2x80x94CR4R5xe2x80x94CR6R7R8)wR9qHzxe2x80x83xe2x80x83(II)
wherein R4, R5, R6, R7, R8, R9, w, z and q have the meaning reported above.
The molar ratio between said organometallic aluminium compound and said water preferably ranges from 1:1 to 100:1; more preferably from 1:1 to 50:1; even more preferably, the Al/H2O molar ratio is 2. The expression xe2x80x9cproduct of reactionxe2x80x9d means the product obtained by contacting the mentioned components.
The molar ratio between said organometallic aluminium compound and said cyclopentadienyl compound, expressed as Al/M molar ration, preferably ranges from 50 to 50000, more preferably from 500 to 5000.
The components of the catalysts of the present invention can be brought into contact by other methods known in the state of the art, such as by first contacting said organometallic aluminium compound with said cyclopentadienyl compound and, thereafter, with water.
According to an embodiment of the invention, water can be gradually added to said organometallic aluminium compound in solution, in an aliphatic or aromatic inert hydrocarbon solvent, such as heptane or toluene. Thereafter, the thus obtained solution is contacted with a solution of said cyclopentadienyl compound in a suitable solvent, such as toluene.
According to another embodiment of the invention, water can be introduced in the monomer or in one of the monomers to be polymerized. In this case, said organometallic aluminium compound and said cyclopentadienyl compound are precontacted before being used in the polymerization.
According to another embodiment of the invention, water can be reacted in a combined form as a hydrated salt, or it can be adsorbed or absorbed on an inert support such as silica.
According to a further embodiment, said organometallic aluminium compound can be allowed to react with boric anhydride and with boric acid.
The catalysts of the present invention can be used on inert supports. This is achieved by depositing said cyclopentadienyl compound, or the product of the reaction thereof with the aluminium compound pre-reacted with water, or said organometallic aluminium compound pre-reacted with water and subsequently said cyclopentadienyl compound, on inert supports such as silica, alumina, styrene/divinylbenzene copolymers, polyethylene or polypropylene.
The thus obtained solid compound, together with further addition of said organometallic aluminium compound, either as such or pre-reacted with water, can be suitably used in gas phase polymerisations.
The catalysts of the present invention can be used in the polymerization reactions of olefins. Therefore, according to further object, the invention provides a process for the polymerization of an olefin in the presence of a catalyst as described above. Olefins which can be polymerized with the process of the present invention are, for instance, xcex1-olefins of formula CH2xe2x95x90CHR, wherein R hydrogen or a C1-C20 alkyl radical.
The catalysts according to the present invention can be conveniently used in the homopolymerization of ethylene, in particular for the preparation of HDPE, and in the copolymerization of ethylene, in particular for the preparation of LLDPE. The LLDPE copolymers which can be prepared have a content of ethylene units ranging from 80 to 99 mol %; their density ranges from 0.87 to 0.95 g/cm3 and they are characterized by a uniform distribution of the xcex1-olefin units along the polymeric chain.
Suitable comonomers in ethylene copolymers are xcex1-olefins of formula CH2xe2x95x90CHR, wherein R is a linear, branched or cyclic C1-C20 alkyl radical, and cycloolefins. Examples of such olefins are propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, allyl-cyclohexane, cyclopentene, cyclohexene, norbomene and 4,6-dimethyl-1-heptene. The CH2xe2x95x90CHR or cycloolefin units are preferably present in the final copolymers in a quantity ranging from 1 to 20 mol %.
Suitable comonomers in said ethylene copolymers are polyenes, in particular conjugated or non-conjugated, linear or cyclic dienes, such as 1,4-hexadiene, isoprene, 1,3-butadiene, 1,5-hexadiene and 1,6-heptadiene.
The catalysts of the invention can be suitably used in propylene homopolymerization, in particular for the production of isotactic polypropylene.
Moreover, the catalysts of the invention can be suitably used in the preparation of elastomeric copolymers of ethylene with a-olefins of formula CH2xe2x95x90CHR, wherein R is a C1-C10 alkyl radical, said copolymers optionally containing minor proportions of units deriving from polyenes.
The saturated elastomeric copolymers obtainable with the catalysts of the present invention preferably contain from 15 to 85 mol % of ethylene units, the complement to 100% consisting of units of one or more xcex1-olefins and/or non-conjugated diolefins able to cyclopolymerise. The unsaturated elastomeric copolymers may also contain, in addition to ethylene and xcex1-olefin units, minor proportions of unsaturated polyene units; the content of unsaturated units can vary from 0.1 to 5% by moles and it is preferably comprised between 0.2 and 2% by moles.
The elastomeric copolymers obtainable with the catalysts of the invention are endowed with valuable properties, such as a low content of ashes and homogeneous distribution of the comonomers along the copolymeric chain.
xcex1-olefins which can be suitably used as comonomers in said elastomeric copolymers are propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene.
Polyenes which can be suitably used in said elastomeric copolymers comprise:
polyenes able to give unsaturated units, such as:
linear, non-conjugated dienes such as 1,4-hexadiene trans, 1,4-hexadiene cis, 6-methyl-1,5-heptadiene, 3,7-dimethyl-1,6-octadiene, 11-methyl-1,10-dodecadiene;
monocyclic diolefins such as, for example, cis-1,5-cyclooctadiene and 5-methyl-1,5-cyclooctadiene;
bicyclic diolefins such as for example 4,5,8,9-tetrahydroindene and 6 and/or 7-methyl-4,5,8,9-tetrahydroindene;
alkenyl or alkylidene norbornenes such as for example, 5-ethyliden-2-norbomene, 5-isopropyliden-2-norbornene, exo-5-isopropenyl-2-norbornene, 5-vinyl-2-norbornene;
polycyclic diolefins such as, for example, dicyclopentadiene, tricyclo-[6.2.1.02.7]4,9-undecadiene and the 4-methyl derivative thereof;
non-conjugated diolefins able to cyclopolymerise, such as 1,5-hexadiene, 1,6-heptadiene, 2-methyl-1,5-hexadiene; conjugated dienes, such as butadiene, 1,3-pentadiene and isoprene.
According to a further embodiment, the catalysts according to the present invention are used in the preparation of cycloolefin polymers. Monocyclic and polycyclic olefin monomers can be either homopolymerized or copolymerized, also with linear olefin monomers. Non limitative examples of cycloolefin polymers which can be prepared with the catalyst of the present invention are described in the European patent applications EP 0 501 370 and EP 0 407 870.
The polymerization processes of the present invention can be carried out in liquid phase, optionally in the presence of an inert hydrocarbon solvent, or in gas phase. Said hydrocarbon solvent can be either aromatic (such as toluene) or aliphatic (such as propane, hexane, heptane, isobutane, cyclohexane and 2,2,4-trimethylpentane).
Polymerization temperature preferably ranges from 0xc2x0 C. to 250xc2x0 C.; in the preparation of HDPE and LLDPE, it is preferably comprised between 20xc2x0 C. and 150xc2x0 C. and, more particularly between 40xc2x0 C. and 90xc2x0 C.; in the preparation of elastomeric copolymers, it is preferably comprised between 0xc2x0 C. and 200xc2x0 C., and more preferably between 20xc2x0 C. and 100xc2x0 C.
The molecular weight of the (co)polymers can be varied simply by varying polymerization temperature, the type or the concentration of the catalyst components, or by using molecular weight regulators, such as hydrogen.
The molecular weight distribution can be varied by using mixtures of different cyclopentadienyl compounds or by carrying out the polymerization in several stages which differ in the polymerization temperature and/or the concentrations of molecular weight regulator.
The polymerization yield depends on the purity of the cyclopentadienyl components (A) in the catalyst. Therefore, said cyclopentadienyl component can be used as such or can be subjected to purification treatments before use.
Particularly interesting results are obtained when the components of the catalyst of the invention are contacted among them before the polymerization. The contact time is preferably comprised between 1 and 60 minutes, more preferably between 5 and 20 minutes. The precontact concentrations for the cyclopentadienyl compound are comprised between 10xe2x88x922 and 10xe2x88x928 mol/l, while for the product of the reaction between the organometallic aluminium compound and water they are comprised between 10 and 10xe2x88x923 mol/l . The precontact is preferably carried out in the presence of a hydrocarbon solvent and, optionally, of small amounts of monomer.
With the exception of tris(2,3,3-trimethyl-butyl)aluminium, tris(2,3-dimethyl-butyl)aluminium and tris(1-menthen-9-yl)aluminium, which were described in Chim. Ind. (Milan) (1976), 58(12), pages 876-7, and Liebigs Ann. Chem., vol. 629, pages 14-19 respectively, the organometallic aluminum compounds of formula (II), as reported above, are new in the state of the art. Therefore, a further object of the present invention is an organometallic aluminium compound of formula (II):
xe2x80x83Al(CH2xe2x80x94CR4R5xe2x80x94CR6R7R8)wR9qHzxe2x80x83xe2x80x83(II)
wherein R4-R9, q, w and z have the meaning reported above, with the exclusion of tris(2,3,3-trimethyl-butyl)aluminium, tris(2,3-dimethyl-butyl)aluminium and tris(1-menthen-9-yl)aluminium.
R4 is preferably methyl and R5 is preferably hydrogen and, according to a particular embodiment of the organometallic aluminum compounds of the invention, R6 has at least two carbon atoms. The variable w is preferably 2 or 3.
It is another object of the present invention a process for preparing compounds of formula (II), as reported above, wherein w is about 3 and R5 is hydrogen, said process comprising reacting:
(a) an alkene of formula CH2xe2x95x90CR4xe2x80x94CR6R7R8, wherein R4, R6, R7 and R8 have the meaning reported above, and
(b) an aluminium compound of formula AlR103, wherein the R10 substituents, the same or different from each other, are hydrogen or alkyl radicals containing a xcex2-hydrogen substituent having a number of carbon atoms lower than that of the alkene (a).
R10 is preferably hydrogen or a C2-C4 alkyl; particularly preferred compounds (b) are triisobutylaluminium and diisobutylaluminium hydride.
Compound (a) must be present in a molar amount at least three times higher than that of compound (b). The reaction can be suitably performed in a hydrocarbon solvent, such as toluene or p-xylene, or for some higher olefins in the absence of added solvents; the temperature is preferably comprised between 100xc2x0 C. and 140xc2x0 C., depending on the solvent and the reactants employed.
The thus obtained compounds of formula (II) generally contain small amounts of the corresponding dialkylaluminium hydride, due to beta-elimination at high temperatures; in fact, the hydride content, besides depending on the nature of the alkyl groups, increases with the thermolysis temperature. Therefore, in order to obtain hydride-free trialkylalurninium compounds, the removal of the solvent and of the excess olefin is carried out with care, at temperatures preferably lower than 50xc2x0 C.